1. Field of the Invention
The present invention relates in general to a fluid-filled elastic mount such as an engine mount for a motor vehicle, and more particularly to such a fluid-filled elastic mount whose vibration damping and isolating characteristics can be controlled by a control signal received from an external controller.
2. Discussion of the Related Art
As a vibration damping/isolating device interposed between two members of a vibration system, there is known an elastic mount in which a first and a second support member fixed to the respective members of the vibration system are elastically or flexibly connected to each other by an elastic body interposed between the two support members. For instance, such an elastic mount is used as an engine mount for mounting a power unit of an automotive vehicle on the body of the vehicle in a vibration damping fashion. In this case, one of the first and second support members of the engine mount is fixed to the vehicle body, while the other support member is fixed to the power unit which includes an engine.
Recent developments to provide a damping/isolating device with enhanced and sophisticated operating characteristics include various types of fluid-filled elastic mounts having a fluid chamber which is partially defined by an elastic body and filled with a non-compressible fluid. Various proposals have been made to control the vibration damping and isolating characteristics of such fluid-filled elastic mounts, by controlling the fluid pressure within the fluid chamber upon application of vibrations, depending upon the specific kind of the applied vibration.
Examples of the fluid-filled elastic mount according to those proposals are disclosed in laid-open Publications JP-A-59-1828 and JP-A-59-1829 of Japanese Patent Applications. In these examples, the fluid chamber is partially defined by an elastic body, and an oscillating plate which is oscillated by an electromagnetic force to control the fluid pressure within the fluid chamber, to thereby control the vibration damping and isolating characteristics of the mount depending upon the types of the input vibrations.
In the known fluid-filled elastic mounts as disclosed in the above-identified publications, an electromagnetic force to drive the oscillating plate is produced by an electromagnetic drive device which uses a permanent magnet and a coil. In this electromagnetic drive device, the magnetic circuit or path formed by the permanent magnet is open, and the region in which the coil is disposed has an insufficient magnetic flux density.
Thus, the conventional electromagnetic drive device is not capable of producing an electromagnetic force sufficient to effectively actuate the oscillating plate when the vibration applied to the fluid-filled elastic mount has a relatively large amplitude and a medium or low frequency. Consequently, the fluid pressure within the fluid chamber cannot be suitably controlled to provide an effective or satisfactory damping effect with respect to such medium- or low-frequency vibrations. Although an increase in the size of the permanent magnet permits an electromagnetic force large enough to effectively actuate the oscillating plate, this solution inevitably results in an undesirable increase in the size and weight of the electromagnetic drive device, leading to an increased cost of manufacture of the mount. In this respect, the above solution is not feasible.
In the light of the foregoing problems, the assignee of the present application proposed a fluid-filled elastic mount having an oscillating plate which is actuated by an improved electromagnetic drive device as disclosed in pending related U.S. patent applications Ser. No. 07/979,934 and U.S. Pat. No. 5,261,649. In the electromagnetic drive device of the proposed mount, a closed magnetic circuit is defined by a stationary permanent magnet and yoke members connected thereto, and an annular moving coil is disposed displaceably in an annular magnetic gap in the closed magnetic circuit, so that the oscillating plate connected to the moving coil is oscillated by the displacement of the moving coil upon energization thereof.
In the fluid-filled elastic mount described above, the magnetic flux density in the annular magnetic gap in which the moving coil is disposed is sufficiently high because of the closed form of the magnetic circuit defined by the permanent magnet and the associated yoke members. This arrangement assures a sufficiently large electromagnetic force produced upon energization of the moving coil, permitting the oscillating plate to be oscillated to provide enhanced vibration damping and isolating characteristics depending upon the specific types of vibrations received.
However, a further investigation of the above electromagnetic drive device by the present applicants has revealed some drawbacks. That is, the moving coil is disposed in an enclosed space or surrounded by the associated members such as the yoke members which define the magnetic circuit. This environment tends to cause difficult dissipation of Joule heat generated by the energized coil, which may cause consequent overheating or burning of the coil, fusion of an external insulator of the coil, consequent short-circuiting of the coil, and disconnection of the lead wires connected to the coil.
Another drawback of the electromagnetic drive device constructed as described above arises from the displacement of the moving coil with the oscillating plate. Namely, repeated movements of the coil occur with repeated movements of the lead wires connected to the coil, and frictional contact of the lead wires with the neighboring members such as the yoke members. Thus, the electromagnetic drive device has some risk of failure due to disconnection of the lead wires.